In FT-IR, a Michelson interferometer has generally been used. The Michelson interferometer is composed of a beam splitter and two light reflecting systems. FIG. 27 schematically shows the structure of a general purpose Michelson interferometer.
In FIG. 27, a Michelson interferometer is provided with beam splitter 01 having a transmittance of 50%, fixed mirror M0 placed so as to face to the beam splitter 01 at a prescribed angle .theta. (45.degree.), and moving mirror M1. The interferometer is fabricated such that when the light which was emerged from measuring light source 02 is converted by collimator 03 into a beam of parallel lights and then enters the beam splitter 01 described above, half of the optical beam is reflected by beam splitter 01 and reaches fixed mirror M0, remaining half of the optical beam passes through beam splitter 01 to reach moving mirror M1, and each of the beams reached mirror M0 and mirror M1 is reflected by them, reenters beam splitter 01, and then collected at detector 05 through condenser lens 04, respectively.
In this case, the two kind of lights interfere to mutually amplify or attenuate (interference action of light) due to twofold difference (difference in optical path) between the distance L0 from beam splitter 01 to fixed mirror M0 and the distance LI from the beam splitter 01 to moving mirror M1. Thus, when moving mirror M1 is reciprocated in parallel to beam splitter 01, twofold value of the moved distance is plotted as abscissa, and the out put from detector 05 is recorded as ordinate, then the interferogram of measuring light (interference waveform) based on the interference action of the lights described above can be obtained. The interferogram is determined and then subjected to Fourier transformation to obtain a spectrum.
In conventional apparatuses for changing the length of an optical path on a cycle in which the length of an optical path of an optical beam from the point where an optical beam emerges from a beam splitter to the point where the optical beam is reflected by a moving mirror, and finally to the point where the reflected optical beam reenters the beam splitter is changed on a cycle by moving the moving mirror, a linear driving mechanism such as a linear ball bearing or a rotational driving mechanism such as a rotary bearing has been used as mechanism for varying the difference in the optical path described above.
FIGS. 28A to 28D schematically show apparatuses for moving a moving mirror (apparatus for moving a light reflecting member) used in conventional interferometers. FIG. 28A is an illustration of an example of conventional apparatuses using a linear driving mechanism. FIG. 28B is an illustration of a sort of prior art technology using a mechanism by which a moving mirror is reciprocated in a prescribed angular range. FIG. 28C is an illustration of another sort of prior art technology using a mechanism by which a moving mirror is reciprocated in a prescribed angular range in a manner different from that of FIG. 28B. FIG. 28D is an illustration of still another sort of prior art technology using a rotational driving mechanism.
In FIG. 28A, supporting member S which supports moving mirror M1 is slidably supported by a guide member. Rack R formed on supporting member S is engaged with gear G. Gear G is reciprocated with a motor (not shown in the drawing) in the range of a prescribed angle, and thus the moving mirror M1 linearly reciprocates in a direction along the incident light.
In FIG. 28B, supporting member S which supports moving mirror M1 is rotatably connected to the free end of swingable parallel links, A1 and A2. Gear G is attached to rotation axis A1a of one side of the parallel links, A1. Gear G is reciprocated with a motor (not shown in the drawing) in a prescribed angular range, and thus the moving mirror M1 linearly reciprocates in a direction along the incident light.
In FIG. 28C, moving mirror M1, and moving mirror M0' which is a substitute for a fixed mirror are supported at the ends of lever L having rotation axis La. Gear G is attached to the rotation axis La. Gear G is reciprocated with a motor (not shown in the drawing) in the range of a prescribed angle. Thus, the moving mirrors M1 and M0' simultaneously swing, and a difference in the length of optical path (difference in an optical path) from the point where a transmitting light and a reflecting light splitted by beam splitter B are caused to emerge to the points where the lights are reflected by the moving mirrors M1 and M0', and finally to the point where the reflected lights reenter beam splitter B, respectively, is produced.
However, the apparatuses shown in FIGS. 28A to 28C have such problems as follows:
In the conventional technology using any one of the apparatuses shown in FIGS. 28A to 28C, it is necessary to reciprocate gear G in the range of a prescribed angle. Accordingly, it is required to once stop the rotational movement of the gear G every time the motion of the gear comes to both ends in the prescribed angle described above and then start the motion in the opposite direction. Thus, it is difficult to make the moving mirror perform a reciprocation at a high speed by a method wherein rotation of the gear is stopped every time it comes up to both ends of the prescribed angle (a method wherein a moving mirror is once stopped at both ends of its moving range, and then the moving of the mirror is started).
FIG. 28D shows a method which has been conducted by Dr.
Griffith of America who is studying on interferometers intended for high speed moving of moving mirrors (that is, interferometers of which the cycle of increasing or decreasing the length of a reciprocative optical path described above is shortened). In FIG. 28D, moving mirror M1 rotates at a high speed together with supporting member S which supports the mirror M1. At that time, the normal of the mirror surface is inclined toward the rotation axis. During a scanty period of time when the moving mirror described above is in the range of a prescribed rotation angle, light L1 which is incident on moving mirror M1 is reflected by moving mirror M1, reflected by mirror M2, M1, M3, M1, M2, and M1 in turn, and then reentered in a beam splitter (not shown in the drawing). The reincident light which reenters the beam splitter can be obtained only during a scanty period of time when the moving mirror M1 is in the range of a prescribed rotation angle. When the moving mirror is in an angular range outside the prescribed one, measuring becomes impossible.